1. Field of the Invention
The present invention relates to an x-ray tube with self-limitation of the electron flux by saturation, for use in particular in the medical field. The main characteristics of these tubes are resistance to drift of their emission characteristics as a function of their temperature as well as homogeneity of the x-ray illumination produced by all the points of their focus. The aim of the invention is to improve such tubes while guarding against any danger of destruction under the action of overheating of their anode.
2. Description of the Prior Art
In general terms, x-rays are produced by electron bombardment, within a vacuum enclosure, of a target fabricated from material having a high atomic number. The electrons which are necessary for bombardment of said target are liberated by thermoelectronic effect, usually in a helical filament of tungsten, of a cathode placed with precision within a concentration component. The concentration component performs a focusing function at the same time as a Wehnelt function. The target is constituted by the anode of the x-ray tube. In this very conventional type of configuration, the initial velocities of the electrons at the level of the emitter are highly dispersed. Their trajectories therefore have a disordered structure and the focusing system provides a correcting function but does not usually have sufficiently high performance characteristics. In consequence, instead of an impact of bombardment electrons on the target, there is obtained a fairly complicated entanglement of trajectories. This provides the thermal focus of the x-rays with an energy profile which is hardly compatible with good quality of the image.
In recent developments, for example in those described in European patent Application No. 85 106753.8 filed on May 31, 1985, reference is made to a cathode which is no longer constituted by a filament but is constituted by a portion of strip provided for emission of electrons with a flat surface located opposite to the anode. The advantage of employing a flat electron emitter has already been presented prior to this Application. It consists in maintaining a certain cohesion of the electronic charges during their trajectory towards the target. Experience has in fact shown that there is obtained in this case a distribution of electrostatic potential which is conducive to better focusing of the electric charges. The x-ray focus thus obtained accordingly exhibits a practically homogeneous energy profile which has a favorable effect on the quality of the image. The scientific literature records certain experiments which are based on this general principle and in which use is always made of an emitter constructed in the form of a tungsten strip.
However, these strips are systematically attended by problems of thermomechanical strength. It was in fact with a view to solving such problems that the European patent Application cited above was filed. In particular, in spite of all the care and attention devoted to rolling of the strips, these strips are subjected to differential stress phenomena and, as a result of successive heating and cooling within the x-ray tube, acquire a so-called corrugated-sheet appearance. The advantages arising from the use of a flat emitter are then lost.
In addition to these defects, flat emitters or even filament emitters have a disadvantage in that the shape of the energy profile of the focus varies in an uncontrolled manner with the load on the x-ray tube. The tube load corresponds to the x-ray delivery. This delivery is related to the magnitude of the thermoelectronic effect in the cathode at the temperature of this latter. In point of fact, more and more radiology devices are provided with regulating circuits for regulating the tube load. This regulation takes into account the x-ray absorption coefficient of a given patient to be examined, with a view to ensuring that the radiation which passes through the patient is limited to a minimum. As can be understood, this regulation acts on the heating circuit of the cathode. The technique whereby regulation tends to produce action on the high voltage between anode and cathode has been abandoned in view of the fact that, in this technique, the hardness of the x-radiation employed has to be modified while an examination is in progress.
Modification of the tube load is not without effect on the energy distribution of the focus. Particularly in certain situations, by reason of the modification of said tube load, the energy densities attained at certain points of the anode may possibly exceed the thermal densities which are acceptable for this anode, in which case the anode is liable to be destroyed. The phenomena of expansion and compression of useful surfaces of the thermal focus are essentially related to the magnitude of the space charge transported by the electrons before impinging upon the target. It is also necessary to relate the magnitude of said space charge to the high voltage which is necessary for detachment of the electrodes from the cathode.
Consideration could also be given to the possibility of modifying the function of the focusing member as a function of the space charge in order to limit, for example, the destructive effects of an excessively abrupt increase in thermal density of the focus. Apart from the complexity of a control system of this type which cannot be contemplated in the present state of the art, it would be necessary to ensure in addition that this control system is capable of rapidly anticipating thermal drift and thermal density of the focus. This solution is not possible at the present time.
In consequence, in the present state of the art, regulation applied to the tube load automatically produces a variation in x-ray illumination and therefore affects the quality of the resultant images. In the final analysis, the heterogeneous character of the combined effects of the space charge and of the high voltage (of the tube load) does not make it possible to obtain tubes in which at least certain emission characteristics would be controlled irrespective of the load.
The aim of the present invention is to overcome this drawback by proposing a flat emitter device having the advantage of offering a degree of mechanical strength which makes it possible to remove the problems of corrugation mentioned earlier. The solution to the problems of limitation of thermal density along the focus as a function of the load on the tube may accordingly be provided by the installation of said flat cathode within a so-called stair-step focusing member. It has in fact been discovered that self-.regulation of the characteristics of said focus takes place in this case. It is accordingly possible in particular to ensure that the quotient of the electron flow rate by the focal surface area is maintained within limits which can be withstood by the target from a thermal standpoint. The advantage of the solution thus presented is that it is applicable over a wide range of high voltage between the anode and the cathode so that one and the same tube can serve for a number of different applications.